102 HYDROGEN ION CONCENTRATION 



the dissociation of the acetic acid, and the pH values are calculated 

 from the free HCl alone. 



2. As soon as the HCl is neutralized the calculation is the same as in 

 the preceding case for CH3COOH + NaOH. 



The curve shows a marked upward bend which corresponds to the 

 disappearance of free HCl and which occurs at about pH 3.0. An 

 indicator which marks off this pH value by a sharp color change is 

 suitable for the determination of HCl in the mixture, a-dinitro- 

 phenol, for example, is such an indicator, showing its first color change 

 at about pH 3.0. If the titration is then continued with phenol- 

 phthalein, the acetic acid content may also be determined in 

 addition. 



There is no significant difference between a one-color and a two- 

 color indicator. In the case of the latter any observed mixed color 

 consists of the optical mixture of the two individual different colored 

 components. Thus, for intance, when a strongly acid red and a 

 strongly alkaline blue litmus solution are placed one in front of the 

 other so that one looks through both at the source of light, an optical 

 impression is obtained of the same violet tint as would be produced by 

 a litmus solution of an appropriate hydrion concentration. This 

 topic will be more fully discussed in the part on methodology. 



28. The determination of the hydrion concentration, or the esti- 

 mation of the actual acidity 



The determination of the hydrion concentration cannot be achieved 

 by means of any titration method. The standard method used for 

 this purpose is the measurement of the electromotive force developed 

 in a hydrogen concentration chain. Schematically drawn this chain 

 consists of two electrodes or half-elements, each being a piece of 

 platinum saturated with hydrogen, and each of these electrodes is 

 immersed into one of the two parts of a liquid system.^ The two 

 parts of the liquid system are a solution of known [H+] and the 

 unknown solution. The way in which the liquid junction between 

 the solutions is arranged will be discussed in the part on methods. 

 Let us at first assume that the known solution is 1 normal in respect 

 to H-ions, [H+] = 1, or pH = 0. The electromotive force (E. M.F.) 



^ An excellent schematic illustration of the gas chain will be found in W. M. 

 Clark, The Determination of Hydrogen Ions, 2nd edition, Baltimore, 1922, 

 Fig. 13, p. 145. 



